CN111308405B

CN111308405B - Method and device for monitoring a local coil

Info

Publication number: CN111308405B
Application number: CN201911248262.9A
Authority: CN
Inventors: J.尼斯特勒; V.马歇尔; S.比伯; R.库尔思; C.温施; G.马特申科
Original assignee: Siemens Healthineers AG
Current assignee: Siemens Healthineers AG
Priority date: 2018-12-12
Filing date: 2019-12-09
Publication date: 2022-08-09
Anticipated expiration: 2039-12-09
Also published as: US20200191890A1; US11226384B2; CN111308405A

Abstract

The present invention relates to a method for operating a magnetic resonance tomography apparatus and to a magnetic resonance tomography apparatus for carrying out the method. In a step of the method, a transmitter of the magnetic resonance tomography apparatus transmits a predetermined test pulse with a reduced power. In a further step, the magnetic resonance tomography apparatus receives examination pulses with the local coil. The controller compares the received verification pulse to a predetermined impulse response and outputs a warning signal when the received verification signal deviates from the predetermined impulse response.

Description

Method and device for monitoring a local coil

Technical Field

The invention relates to a method for operating a magnetic resonance tomography system, wherein the magnetic resonance tomography system has a controller, a local coil and a transmitter for transmitting excitation pulses, and the function of the local coil is checked by means of the method.

Background

A magnetic resonance tomography apparatus is an imaging apparatus which aligns the nuclear spins of an examination subject with a strong external magnetic field and excites the nuclear spins to precess around the alignment by an alternating magnetic field in order to image the examination subject. Precession of the spins, or return from the excited state to a state with lower energy, in response, generates an alternating magnetic field that is received by the antenna.

A spatial coding is applied to the signals by means of the gradient magnetic fields, which then makes it possible to correlate the received signals with the volume elements. The received signals are then analyzed and a three-dimensional imaging representation of the object under examination is provided.

In order to achieve a Signal-to-Noise Ratio (SNR) which is as good as possible, it is attempted to arrange the antenna coil for reception as close as possible to the patient. This is achieved by so-called local coils, which are usually connected to the magnetic resonance tomography apparatus by cable connections. However, due to the high field when exciting the nuclear spins, a faulty local coil can also cause damage to the patient when the protection device fails.

Disclosure of Invention

The object of the invention is therefore to make the use of local coils safer.

This object is achieved by a method for operating a magnetic resonance tomography system according to the invention and by a magnetic resonance tomography system according to the invention.

The method according to the invention is provided for operating a magnetic resonance tomography system, wherein the magnetic resonance tomography system has a controller, a local coil and a transmitter for transmitting test pulses.

In a step of the method according to the invention, the magnetic resonance tomography apparatus transmits a predetermined test pulse with reduced power by means of a transmitter via a transmitting antenna. It is conceivable here for the transmitter of the magnetic resonance tomography apparatus to be designed for transmitting excitation pulses with a power of several hundred watts to several kilowatts and also for generating predetermined test pulses with a power of a fraction of a watt to several watts and with sufficient linearity and frequency purity, for example in such a way that the circuit is sufficiently linear or that a plurality of transmission modules can be activated individually for different power ranges. It is also conceivable to have an attenuation unit as part of the transmitter, which reduces the generated signal to within the desired power range. Finally, a special transmitter for generating the test pulses only can also be provided in the magnetic resonance tomography apparatus.

The transmitting antenna can be, for example, a body coil of a magnetic resonance tomography apparatus. Separate transmit antennas are also contemplated. Here, a power of less than 0.1 watt, 1 watt or 5 watts is considered as reduced power. The reduced power can likewise be given by the amplitude of the test pulse formed at the transmitting antenna, the effective voltage being less than 0.5V, 1V, 5V or 15V.

In a step of the method according to the invention, the local coil receives a test pulse. The received test pulses can be preprocessed in the local coil, for example amplified and converted into frequency, or digitized. The local coil is usually in signal connection with a receiver of the magnetic resonance tomography apparatus, which receiver undertakes further analysis.

In a further step, the local coil and/or the receiver forwards the received test pulse to the controller, which compares the test pulse with a predetermined impulse response. The predetermined impulse response may be a threshold or desired value against which the controller compares the verification pulse. It is also possible to use the check pulse received at an earlier point in time as a comparison value. It is also conceivable to specify a time pulse profile which has time-dependent values, for example in the form of an equation or a table. Here, the comparison may mean whether these values are equal to, less than, or greater than the desired values. The comparison may also mean that a maximum distance that is not allowed to be exceeded is determined using the metric. For example, it is conceivable to form the sum of the squares of the distances for all time points.

In a further step, the controller outputs a warning signal when the received verification signal deviates from a predetermined impulse response. This may be set, for example, when a threshold value is exceeded or fallen below, or when the sum of the squares of the distances explained above exceeds a threshold value. The warning signal may, for example, cause the controller to inhibit subsequent testing in order to avoid endangering the patient.

In an advantageous manner, the method according to the invention allows the function of the local coil to be monitored without expensive additional instrumentation, so that the function and safety of the local coil connected to the magnetic resonance tomography apparatus is ensured at any time.

The magnetic resonance tomography apparatus according to the invention shares the advantages of the method according to the invention.

The invention also provides other advantageous embodiments.

In a possible embodiment of the method according to the invention, the predetermined check pulse has an amplitude of 0 volt. It is understood that for example the transmitter is connected to an antenna and is set up as when an excitation pulse is transmitted, but the transmitter does not obtain an input signal or an input signal with a constant amplitude. Therefore, the transmitter does not actively transmit a high frequency, but the transmitter is electrically connected to the antenna and transmits background noise of the transmitter. The predetermined impulse response has an elevated noise level. In the case of such a test pulse with amplitude, a correspondingly higher noise level is expected on the receiving side. If the noise does not increase the desired value, the preamplifier may be faulty, for example.

Advantageously, a test pulse with a low amplitude can be used to test the basic function of the local coil with a minimum of HF emissions.

In a conceivable embodiment of the method according to the invention, the amplitude of the test pulse increases from a lower starting value to a higher end value over the duration of the test pulse. Here, for example, a power of less than 0.1 watt, 0.5 watt or 1 watt is considered as a low starting value. The lower starting value can likewise be given by the amplitude of the test pulse formed at the transmitting antenna, wherein the effective voltage is less than 0.1V, 0.5V or 1V, for example. The amplitude increases in the time course of the test pulse to a higher end value. For example, powers greater than 1 watt, 2 watts, 5 watts or 10 watts are considered as higher end values. The higher end value can also be given by the amplitude of the test pulse formed at the transmitting antenna, the effective voltage being greater than 1V, 2V, 5V or 10V, for example. The course can increase monotonically, for example linearly over time. It is also conceivable, however, for the increase to be exponential, logarithmic, phased or with a drop in amplitude in the course of time.

In an advantageous manner, the increase in amplitude allows the interruption at lower power already before further damage can occur in the case of a faulty behaviour of the local coil having been identified. A linear increase may enable a simple comparison with a predetermined test response. The increase in the index in turn allows a greater functional range to be monitored in a short time. Conversely, for diodes in the local coil, the logarithmic increase results in a linear increase, so that the function of the PIN diode or the protection diode can be easily checked.

In a possible embodiment of the method according to the invention, the local coil has a detuning device. The detuning means have a variable capacitance or a switchable capacitance or an otherwise variable capacitance, for example in the form of a PIN diode. The detuning device is designed to tune the resonance frequency of the local coil in the active state to a frequency that is not equal to the excitation pulse for the nuclear spins, so that no dangerous voltages are induced in the local coil by the excitation pulse. During the receiving step, a detuning means of the local coil is activated.

In an advantageous manner, the function of the detuning device results in a smaller amplitude of the received test pulse, which amplitude can be verified by means of a correspondingly smaller predetermined impulse response, and thus the function of the detuning device is ensured.

In a conceivable embodiment of the method according to the invention, the predetermined impulse response has a threshold value. Here, a single constant value of the amplitude of the impulse response is considered as a threshold. Depending on the settings of the local coil and the test pulse, exceeding or falling below a threshold value can indicate a defective function of the local coil. For example, if the detuning device is activated during the check pulse, exceeding the threshold value can indicate a faulty functioning of the detuning device. This also makes it possible to identify a defective function of the protective device. Conversely, a threshold value below can indicate, for example, a malfunctioning of the input amplifier.

In an advantageous manner, the threshold value provides a simple possibility: the function of the local coil is verified by a simple circuit or in a digital implementation by a comparator.

In a possible embodiment of the method according to the invention, the predetermined impulse response is proportional to the test pulse. In other words, at any point in time of the test pulse, the quotient of the test pulse and the value of the temporally associated impulse response is substantially constant. Deviations of less than 20%, 10%, 5% or 1% of the average quotient are considered here to be substantially constant.

In the receiving action, the local coil must first of all exhibit a behavior which is as linear as possible, so that it can be verified in a simple manner by checking the ratio of the pulse to the predetermined impulse response and the received signal which is compared therewith.

In one embodiment, the method according to the invention provides a detuning device for checking an antenna coil of a magnetic resonance tomography apparatus. For the detuning means, there is the possibility of the latter being activated in a targeted manner by means of a control signal. The control signal may be a dc voltage or a dc current applied to the detuning means as is common in PIN diodes or switches. However, the control signal may also be a further analog or digital signal, which in the control device of the detuning device leads to a detuning of the antenna coil.

In particular, however, the activation of the detuning means by signals, in particular high-frequency signals induced in the antenna coil (which signals themselves may cause danger to the patient due to the induced voltage), is not considered to be an activation of the detuning means in the sense of the present invention, as is the case, for example, in passive protection devices or zener diodes. Preferably, the detuning means cause a shift of the resonance frequency of the antenna coil, wherein high frequency signals can still be received by the connected receiver. The detuning means can be realized, for example, by a diode with a variable capacitance or a switchable inductance or capacitance in a parallel or series resonant circuit of the antenna coil. However, it is conceivable that the detuning means also have passive components that limit the induced voltage or current. For example, detuning diodes are conceivable, which are detuned by the induced rectified voltage in the absence of an external control signal; or a diode of a cross-bar switch or a zener diode, which short-circuits the induced voltage above a threshold value.

In a step of the method, the detuning means of the antenna coil are activated, for example, by applying a voltage of a current or a further control signal to the control means of the antenna coil.

In a further step, the receiver receives a first reception signal of the antenna coil with the detuning device activated. In this case, high-frequency current signals and/or voltage signals are considered as received signals, which are processed and/or digitized analog or analog to the receiver. In particular, the received signal has information about amplitude, phase and/or spectral distribution. The receiver may be a dedicated receiver for checking the detuning means, but is preferably a receiver of the magnetic resonance tomography apparatus, which is also used for receiving magnetic resonance signals for imaging by the magnetic resonance tomography apparatus.

In a further step, a second reception signal of the antenna coil is received with the receiver. In this case, as the invention is subsequently implemented separately, the detuning device is activated or deactivated according to an embodiment of the method. The already mentioned applies with respect to the received signal and the receiver.

In a further step, the test controller compares the first received signal with the second received signal. The examination controller can be, for example, a controller of a magnetic resonance tomography apparatus or a controller of an image analyzer or a dedicated processor. The comparison of the first received signal with the second received signal can be, for example, a difference or quotient and a comparison with a predetermined difference or quotient or a relationship of average or maximum amplitude or energy. Preferably, the comparison is not limited to precise values, but may also be defined by a value range which is, for example, at most 5%, 10%, 20% or 50% wide of one of the two values of the first or second received signal, or in the case of a quotient deviates by a factor of less than 0.1, 0.2 or 0.5 with respect to the value 1. Further mathematical calculations of these relationships in the context of the comparison are also conceivable. The invention is given subsequently by way of example of embodiments thereof.

If the result of the comparison does not correspond to the predetermined value range, the controller outputs a warning signal to the user via the output device and/or interrupts further image acquisition.

It is conceivable, for example, for the temporal change, for example the attenuation, of the input signal to be dependent on the detuning device being activated, and thus the function of the detuning device can be inferred by comparing the two received signals. In an advantageous manner, the method according to the invention makes it possible to achieve a fast check of the detuning means with components of the magnetic resonance tomography apparatus.

In a possible embodiment of the method according to the invention, the method has the step of deactivating the detuning means. The step of receiving the first receive signal is performed with the detuning means activated, and the step of receiving the second receive signal is performed with the detuning means deactivated. In the comparing step, the noise level of the first received signal is compared with the noise level of the second received signal. For example, the amplitude or energy of the noise may be compared by forming a difference or a quotient. It is also conceivable to express the result logarithmically over a large dynamic range. It is also conceivable to take into account the spectral distribution of the energy of the noise, which is caused by the change of the resonance frequency by the detuning means.

Comparing the two received signals with the detuning means activated and with the detuning means deactivated allows the function of the active components of the detuning means to be checked in a simple manner.

In a conceivable embodiment of the method according to the invention, the transmitter of the magnetic resonance tomography apparatus transmits the small signal during the step of receiving the first and second receive signals. The amplitude of the test pulse is low enough to exclude subsequent damage to the local coil or danger to the patient in the event of a local coil failure and not to override the receiver, considered a small signal. Therefore, the small signal or test pulse must have a lower power or reduced power. Here, a power of less than 0.1 watt, 1 watt or 5 watts is considered as reduced power. The reduced power can likewise be given by the amplitude of the test pulse formed at the transmit antenna, wherein the effective voltage is typically less than 0.5V, 1V, 5V or 15V.

For example, a separate signal source may be provided as a transmitter for generating the small signal.

By means of the small signal, without over-controlling the receiver, it is advantageously possible with the method according to the invention to identify small differences, for example slight attenuations or changes in the spectral distribution, in the comparison of the first received signal with the second received signal.

In a possible embodiment of the method according to the invention, a transmitter for generating excitation pulses of the magnetic resonance tomography apparatus is provided to generate the small signal. For this purpose, the transmitter has a switchable damping device between the signal generator and the power output stage, which damping device is designed to damp the input signal of the power output stage during the receiving step, so that the transmitter does not over-control the receiver. For this purpose, the transmitter must have a correspondingly linear circuit technology in order to be able to generate a correspondingly small signal strength proportionally and with sufficiently small distortions and noises. For example, switchable attenuators may be provided to reduce the input signal by 40dB, 60dB, 80dB, 100dB or more, so that the transmitter also produces a correspondingly reduced output signal. However, it is also possible to provide a corresponding damping element on the output side, thereby reducing the requirements on the linearity of the power output stage.

In an advantageous manner, the use of a transmitter for generating the excitation pulses in the detection of detuning means makes it possible to integrate the test easily into existing hardware.

In a conceivable embodiment of the method according to the invention, a transmitter for generating excitation pulses of the magnetic resonance tomography apparatus is provided, which transmitter is also used for generating the small signal. Here, the power output stage is not fed with the input signal during the steps of receiving the first receive signal and receiving the second receive signal. In other words, the generation of the excitation signal is interrupted or a signal having a constant output value is generated. For example, it is conceivable to interrupt the connection between the signal generator and the power output stage or to connect the input of the power output stage to a constant potential. The power output stage then generates noise at the output, which is transmitted as a small signal in the test method.

The use of noise at the power output stage also makes it possible to generate and transmit small signals with the transmitter according to the invention without modifying the transmitter, and thus simplifies the implementation of the checking method according to the invention in particular.

In a possible embodiment of the method according to the invention, a signal source is arranged as a transmitter of the small signal within the patient channel during the step of receiving the first received signal and the second received signal. The signal source transmits a small signal, wherein the power of the small signal varies by a predetermined parameter between the step of receiving the first received signal and the step of receiving the second received signal. Preferably, the frequency of the high frequency alternating field is equal to or in a range close to the larmor frequency.

In an advantageous manner, the predetermined parametric variation of the small signal gives the possibility of identifying the small signal and/or of eliminating the background signal. Furthermore, the validity of the passive elements of the detuning means can be checked, for example, by the nonlinear behavior of the received signal.

In a conceivable embodiment of the method according to the invention, an active transmitter of the magnetic resonance tomography apparatus generates the signal. Here, the following transmitters or oscillators are considered as active transmitters: these transmitters or oscillators not only reflect the incident high frequencies or store them intermediately and transmit them as high-frequency currents in the resonant circuit or in the excited quantum state, but also generate high-frequency signals from the currents fed by the energy source via the electrical conductors. This signal is a small signal that does not override the control receiver. The verification controller controls the transmitter to change the power in one step. For example, the verification controller may increase or decrease amplification in the transmitter between receiving the first received signal and receiving the second received signal. It is also conceivable for the test controller to vary the amplitude of the input signal of the transmitter.

By means of the test controller, the signal can be varied in a predetermined manner, so that the amplitude can be set in an advantageous manner, at which the detuning means and its correct function can be observed in different ways, for example by means of non-linearity or predetermined attenuation or energy consumption.

In a possible embodiment of the method according to the invention, the transmitter is a passive signal source, for example a resonator or an object with nuclear spins, which is temporarily excited by a high-frequency excitation signal of the magnetic resonance tomography apparatus to emit an alternating magnetic field, which then decreases exponentially over time in a predetermined manner by damping. In this way, a high-frequency signal or a magnetic resonance signal that falls with time is generated as a small signal between the step of receiving the first reception signal and the step of receiving the second reception signal.

In an advantageous manner, the passive signal source as transmitter does not require or only requires minimal changes to the magnetic resonance tomography apparatus, and the test signal can still be provided in the frequency range of the desired larmor frequency and with a small amplitude suitable for the receiver.

In an embodiment of the invention, a magnetic resonance tomography apparatus is provided for carrying out the method according to the invention. For this purpose, the magnetic resonance tomography apparatus has a controller, a local coil, a transmitter and a transmitting antenna for transmitting excitation pulses. Preferably, the transmitting antenna is a body coil, in particular in the form of a so-called birdcage.

Furthermore, the transmitter has a small signal path which enables the small signal to be fed directly into the transmit antenna. For example, the transmitter for providing the excitation pulses may have a signal generator with an oscillator, a mixer or a modulator, wherein the fundamental frequency may be modulated with a modulation signal in amplitude and frequency direction. This can be done in an analog or digital manner, where digital-to-analog conversion is performed by an AD converter before amplification by the power output stage. As a small signal path, the magnetic resonance tomography apparatus according to the invention has a signal connection between the signal generator and the transmitting antenna (for example a birdcage). Preferably, the small signal has a level less than +30dBm, 10dBm, 0dBm or-10 dBm.

It is also conceivable here to provide two signal generators or one signal generator with two signal outputs with different phases, preferably offset by 90 degrees. Thus, a small signal connection over a dual channel may also utilize a birdcage antenna to provide cyclic modulation with reduced levels.

In an advantageous manner, small signals for verification purposes can therefore be made available with little overhead with a low noise background.

In a possible embodiment of the magnetic resonance tomography apparatus according to the invention, the small signal path provides a direct connection between the high-frequency source of the transmitter and the transmit antenna while bypassing the power input stage. For example, it is contemplated that an electronic or mechanical switch may effect switching of the signal path under the control of the controller.

Advantageously, bypassing the power amplifier makes it possible to use the power amplifier for linearization in the magnetic resonance tomography apparatus according to the invention unchanged, in particular without expensive and efficiency-reducing measures.

In a conceivable embodiment of the magnetic resonance tomography apparatus according to the invention, the small signal path has a small signal amplifier and/or an attenuation element. Preferably, the small-signal amplifier and/or the attenuating element can be changed or switched in its attenuation or amplification by the controller.

In an advantageous manner, the switchable amplification can, for example, reduce the signal to the noise limit or increase it to the nonlinear region of the diode, so that further functions of the local coil can be checked.

In a possible embodiment of the magnetic resonance tomography apparatus according to the invention, the controller is designed to transmit a predetermined test pulse with reduced power via the small signal path and the transmitting antenna and to receive the test pulse via the local coil. The controller is furthermore designed to compare the received test pulse with a predetermined impulse response. The predetermined impulse response can be stored, for example, as a value, a value range or a value table, or can be determined computationally by a function or a relationship in the control unit as a function of the test impulse. If the received test pulse deviates from the predetermined impulse response, for example because the amplitude is too large due to a defect of the detuning device or too small in the case of a break in the signal line, the controller outputs a warning signal, which is output to the user, for example via an operating interface, or the image acquisition is immediately stopped.

In an advantageous manner, the small signal path improves and expands the examination possibilities of the magnetic resonance tomography apparatus, thereby increasing the safety and reliability.

In an embodiment of the magnetic resonance tomography apparatus, it is also conceivable for the magnetic resonance tomography apparatus to have a directional coupler and a test switch, which is in signal connection with the small signal path and the directional coupler. The controller can then be designed such that, in an additional test step, the controller feeds a test signal from the small signal path through the test switch into the directional coupler and compares the output signal of the directional coupler with a predetermined value. Thus, for example, a drift of the directional coupler can be detected by a deviation in the amplitude.

In an advantageous manner, a small signal path with a test switch also makes it possible to test the directional coupler, thus ensuring proper functioning of the SAR monitoring and safety of the patient.

In a possible embodiment, the magnetic resonance tomography apparatus according to the invention has a test device which uses a test controller to test the detuning device of the antenna coil of the magnetic resonance tomography apparatus. The test controller can be a dedicated processor or logic module, but can also be a processor of the magnetic resonance tomography apparatus, which takes over the tasks described by the program in addition to the image acquisition. The test controller is designed to activate the detuning means of the antenna coil; receiving a first received signal with a receiver; receiving a second received signal with a receiver; comparing the first received signal with the second received signal; and outputting a warning signal if the result of the comparison does not correspond to the predetermined relationship of the first received signal relative to the second received signal. The warning signal may be output to a user, for example on a display, or to a controller of the magnetic resonance tomography apparatus, to stop further image acquisitions.

Drawings

The above described features, characteristics and advantages of the present invention and the manner of attaining them will become more apparent and the invention itself will be better understood by reference to the following description of embodiments taken in conjunction with the accompanying drawings.

In the drawings:

fig. 1 shows a schematic representation of a magnetic resonance tomography apparatus with a local coil according to the invention;

FIG. 2 shows a schematic flow chart of an exemplary embodiment of a method according to the present invention;

fig. 3 shows a schematic illustration of an examination apparatus of a magnetic resonance tomography apparatus according to the invention;

fig. 4 shows a schematic flow chart of an exemplary embodiment of a method according to the present invention.

Detailed Description

Fig. 1 shows a schematic illustration of an embodiment of a magnetic resonance tomography apparatus 1 with a local coil 50 according to the invention.

The magnet unit 10 has a field magnet 11, and the field magnet 11 generates a static magnetic field B0, which is used to align the nuclear spins of the sample or patient 100 in the recording region B0. The recording region is characterized by a very homogeneous static magnetic field B0, wherein the homogeneity relates in particular to the magnetic field strength or the amount of the magnetic field. The recording region is almost spherical and is arranged in a patient channel 16, which extends through the magnet unit 10 in the longitudinal direction 2. The patient bed 30 can be moved in the patient tunnel 16 by means of a movement unit 36. Typically, field magnet 11 is a superconducting magnet, which can provide a magnetic field with flux densities up to 3T, and even higher with the latest devices. However, for smaller field strengths, it is also possible to use permanent magnets or electromagnets with normal electrically conductive coils.

Furthermore, the magnet unit 10 has a gradient coil 12 which is designed to superimpose a variable magnetic field for the magnetic field B0 in three spatial directions for spatial differences of the acquired imaging region in the examination volume. The gradient coils 12 are typically coils of normally conductive wire, which can generate mutually orthogonal fields in the examination volume.

The magnet unit 10 likewise has a body coil 14, the body coil 14 being designed to inject high-frequency signals fed via a signal line into the examination volume and to receive resonance signals emitted by the patient 100 and to output them via the signal line.

The control unit 20 provides the magnet unit 10 with different signals for the gradient coils 12 and the body coil 14 and analyzes the received signals.

The control unit 20 therefore has a gradient controller 21, the gradient controller 21 being designed to supply the gradient coils 12 with variable currents via supply lines, which provide the desired gradient fields in the examination volume in a time-coordinated manner.

Furthermore, the control unit 20 has a radio-frequency unit 22, the radio-frequency unit 22 being designed to generate radio-frequency pulses having a predetermined temporal profile, amplitude and spectral power distribution in order to excite magnetic resonance of the nuclear spins in the patient 100. In this case, pulse powers in the kilowatt range can be achieved. The excitation pulses can be transmitted into the patient 100 by the body coil 14 or by a local transmitting antenna.

The controller 23 communicates with the gradient controller 21 and the high frequency unit 22 via a signal bus 25.

A local coil 50 is arranged on the patient 100, which is connected to the radio-frequency unit 22 and its receiver via a connection line 33.

A faulty local coil 50 can be particularly dangerous due to the close proximity to the patient if, for example, the means for detuning the local coil 50 fails and an excessively high voltage and/or current is induced in the local coil during the excitation pulse. The local coils 50 are also particularly prone to failure due to long-term movement, insertion and removal of the connections. It is therefore advantageous to use the magnetic resonance tomography apparatus to test at least before each image acquisition with regard to the basic function of the local coil 10 in the configuration provided for the image acquisition.

For the test according to the invention, provision is first made for the controller 23 to cause the high-frequency unit 22 to emit a test pulse having a predetermined characteristic. Depending on the type of test pulse, different defects in the local coil 50 can be identified or excluded therefrom, as described in more detail below. However, all test cases have in common that the amplitude of the test pulses is sufficiently low to preclude subsequent damage to the local coil 50 or risk to the patient in the event of a failure of the local coil. Therefore, the verify pulse must have a lower power or reduced power. Here, a power of less than 0.1 watt, 1 watt or 5 watts is considered a reduced power. The reduced power can likewise be given by the amplitude of the test pulse formed at the transmit antenna, with an effective voltage typically less than 0.5V, 1V, 5V or 15V.

In order to generate test pulses with such a reduced power, the high-frequency unit 22 must be designed to generate predetermined test pulses in the power range or lower, in addition to excitation pulses in the power range of several hundred watts to kilowatts, for example by flexibly coupling power stages in different power ranges using correspondingly linear circuit technology or by providing a sufficiently strong attenuation of the high output signal. It is also conceivable for the high-frequency unit 22 to have a separate test transmitter 60 for generating the test pulses.

It is also possible that the control signal for the power stage feeds the small signal directly to the transmitting antenna as a check transmitter 60 via a small signal path or a bypass to the power stage of the high-frequency unit 22. Preferably, this small signal path may be switched on by a mechanical or electronic switch to protect the test transmitter 60 from damage during the excitation pulse.

If the control signal for the power stage should have too little power to function as a test transmitter, it is also conceivable for the small-signal path to have a small-signal amplifier which has sufficient linearity even with low power. Conversely, it is also conceivable to provide an attenuation element in the small signal path in the case of excessive power, which attenuation element further reduces the power.

The test pulses are transmitted via a transmitting antenna, for example the body coil 14, into the patient channel 16, in which the local coil 50 is arranged depending on the application. However, a separate transmitting antenna 70 for the test pulses is also conceivable, as is shown by way of example in fig. 1.

The controller 23 is also designed to receive the examination pulses via the radio-frequency unit 22 and the local coil to be examined. In a possible implementation, the signal of the received test pulse is digitized and written into a memory, where it is available for further processing or for analysis by the controller 23.

Finally, the controller 23 compares the received verification pulse with a predetermined impulse response. Here, the comparison may be a magnitude comparison with a threshold; or a comparison of the profile with a profile of a predetermined impulse response, which is predefined in a table or by a function. Examples are given below for each checking case.

If the received check pulse deviates from a predetermined magnitude, the controller 23 outputs a warning signal. A warning signal may be output to the operator via the display or a subsequent image acquisition may be immediately interrupted or prevented to avoid endangering the patient 100.

The deviation can be a threshold value being exceeded or undershot or an excessive spacing between the received test pulse and the predetermined pulse response. The spacing can be, for example, the sum of the squares of the differences between the test pulses received within the duration of the test pulses and the predetermined impulse response.

In this case, different functions of the local coil 50 can be checked as a function of the shape and amplitude of the check pulse and of the predetermined check response.

The basic function of receiving with high sensitivity by means of local coils can already be achieved, for example, by transmitting test pulses with an amplitude of 0. It is to be understood that although the transmitter is connected to the transmission antenna for transmitting the test pulses, no periodic test signal is generated, which is separated from the output stage by, for example, an oscillator. Then, only the noise signal of the transmitter is applied to the transmit antenna, which is received by the local coil 50 and leads to a significantly higher noise level. The noise level is increased by a factor of more than 10, 20, 50 or 100 relative to the noise level without coupling the transmitter to the transmit antenna. In this case, the desired impulse response may be, for example, a threshold value, which corresponds to the background noise of the local coil multiplied by the factor. In this case, exceeding the threshold value indicates a function of the local coil with sufficiently high sensitivity.

This checking with amplitude can be supplemented by a further step in that a check pulse with an amplitude of 0 is generated, while the detuning means of the local coil 50 are activated. For example, for a body coil, the noise signal is spectrally concentrated at the larmor frequency due to the resonance of the transmitting antenna. If the detuning means of the local coil are activated and their resonance frequency is shifted from the larmor frequency of the magnetic resonance tomography device, the received noise signal also drops accordingly. In this verification step, the predetermined impulse response is greater than the background noise of the receiver, but smaller by a factor of, for example, 2, 5, 10, 50, or 100 than without the detuning. That is, if the signal received by the local coil falls below a correspondingly small threshold value in the case of activation of the detuning means, the correct functioning of the reception path and of the detuning means can be concluded in conjunction with the preceding checking steps. Conversely, if the threshold value is not exceeded with amplitude 0 or is not undershot with an active detuning device, the controller 23 can conclude the respective malfunction of the receiver and/or the detuning device. The controller 26 may then output a warning signal to the user acoustically or via a display and/or interrupt further image acquisition for safety reasons.

In a further possible embodiment of the method according to the invention, the amplitude of the test pulse increases from a lower starting value to a higher end value within the duration of the test pulse. It is conceivable that the amplitude increases linearly in the form of a ramp. However, other increasing curve forms are also conceivable. This can also be approximated by a number of increments, point by point or step by step. In principle, descending curves or sequences are also conceivable. However, the addition is preferred because problems with the local coil can be identified at lower power before subsequent damage can result at higher power.

For increasing trends, different predetermined impulse responses may be expected depending on the equipment, control or failure of the local coil. For a local coil 50 without interference, the following signals are initially expected as an impulse response: due to the high sensitivity of the local coil 50 to the MR signal, this signal increases very rapidly and goes into saturation due to the over-control.

Conversely, if the detuning means of the local coil 50 are activated by the controller 23 and the sensitivity is reduced, the received signal is expected to increase as an impulse response from the controller 23 in proportion to the test pulse, so that the quotient of the received test pulse and the expected value of the impulse response is constant. If the check pulse has a ramp-like increase, for example, the expected value of the predetermined impulse response is also a ramp.

Conversely, if, in the absence of detuning, the received signal increases first for a ramp-like test pulse to enter saturation at higher transmit levels, it is suspected that the detuning means is in a sense that it is faulty, for example that the detuning diode is permanently switched into a partially conducting state.

In this case, it is also conceivable to have a smaller increase or a proportional increase of the quotient of the received test pulses compared to a measurement with an activated detuning device. If the comparison of the received test pulses corresponds to such a predetermined impulse response, this may indicate, for example, a malfunction of the protective device of the local coil 50 with respect to an induced overvoltage (permanent on-state) and a warning signal may be output by the controller 23 and/or further imaging may be disabled accordingly.

In principle, a plurality of the described test pulses can also be combined into a single test pulse having different levels and a local coil activity synchronized therewith. In addition to the accelerated execution of the test, more information about the state of the local coil 50 can be obtained by the signals received during the changeover and the comparison with the expected impulse response, and possible malfunctions can be warned or interrupted.

In this case, it is always conceivable according to the invention for the illustrated fault situation that a continuous progression, for example in the form of a ramp, is replaced by a plurality of discrete test levels in the test pulse. In principle, the sequence can also be changed, so that monotonically increasing amplitudes are not necessarily arranged in succession as test pulses.

In a conceivable embodiment of the magnetic resonance tomography apparatus according to the invention, the controller 23 transmits a predetermined test pulse with reduced power via the small signal path and the transmitting antenna. In this case, a predetermined, preferably periodic high-frequency signal with a frequency equal to or close to the larmor frequency of the magnetic resonance tomography apparatus, which is located above the noise signal of the output stage, for example by more than 6dB, 12dB, 24dB or 40dB, is considered as a test pulse of reduced power. In a preferred embodiment, the test pulse is constant here, i.e. it has a constant frequency and/or amplitude.

The test pulse is then received again by the local coil. In this case, the controller 23 compares the received test pulse with a predetermined value or value range. For example, in the case of an activated detuning device, a very small amplitude value of the received test pulse is expected, whereas in the case of an deactivated detuning device, a very high input value is expected, for example, 20dB, 40dB or more than 60dB greater than the input value with the detuning device activated. The small signal path allows the test pulse to be controlled very precisely in a lower power range, which is lower than the linear range of the power output stage, and thus also the fault behavior of the different detuning means, active and passive, is recognized and differentiated.

If the magnetic resonance tomography apparatus has a directional coupler for monitoring the transmission signals, faults can also occur in the directional coupler, which in turn endangers the safety of the patient. The power decoupled by the directional coupler is very small compared to the power fed to the body coil 14 during the excitation pulse. Therefore, a very low power check pulse is also required for checking the transmission monitoring. In a conceivable embodiment, the magnetic resonance tomography apparatus according to the invention therefore has a test switch which is in signal connection with the small signal path and the directional coupler. In this way, the controller 23 can feed a test signal from the small signal path into the receive signal path of the directional coupler by means of the test switch and compare the output signal of the directional coupler with a predetermined value. In this way, a malfunction of the directional coupler and of an evaluation circuit connected to the directional coupler can be identified.

Fig. 2 shows a schematic flow diagram of an exemplary method according to the invention.

In step (S10), the transmitter transmits a predetermined check pulse with reduced power. This can be done, for example, by means of an antenna. The same applies as already mentioned above with respect to the reduced power. This may be, for example, a noise signal of the power amplifier or a signal fed through a small signal path.

In step (S20), the local coil 50 receives the verification pulse and forwards a signal to the controller 23. It is conceivable here for the local coil 50 to arrange itself in order and/or digitize the received test pulses. It is also conceivable, however, for this to be done by a receiver of the magnetic resonance tomography apparatus 1 and for this receiver to forward the signal to a controller for analysis. It is also conceivable, for example, to store the received test pulses as a sequence of values in a memory which is accessed by the controller 23.

In step (S30), the controller 23 compares the received verification pulse with a predetermined impulse response. As already explained, the comparison can be a comparison with a threshold value or a determination of the distance of the received test pulse from the predetermined impulse response. A predetermined permissible value range for the impulse response or the received test signal is also conceivable.

In step (S40), if the received test signal deviates from the predetermined impulse response, the controller 23 outputs a warning signal, for example, acoustically or on a display, or interrupts the image acquisition process of the magnetic resonance tomography apparatus by means of the warning signal. Depending on the test pulse and the fault behavior to be checked, the deviation may be above or below a threshold value or an excessive deviation from a predetermined impulse response. It is also conceivable that the predetermined impulse response lies within or outside a predetermined range of values of the impulse response.

The elements of the checking device required for an embodiment of the method according to the invention are schematically shown in fig. 3.

The local coil 50 has a detuning device 51 in order to prevent excessive currents and/or voltages from being induced in the antenna coil of the local coil 50 during the excitation pulse for the nuclear spins, which could damage the local coil and could also endanger the patient, in particular by voltage or thermal expansion. The detuning means 50 preferably have a passive detuning 53 and an active detuning 52 for this purpose. Passive detuning is for example realized by two diodes connected in anti-parallel, so that in case the induced alternating voltage exceeds the threshold voltage of the diodes, the diodes become conductive and an additional LC element is connected in series into the antenna coil, thereby changing the resonance frequency and limiting the induced voltage.

The active detuning 52 is realized by a PIN diode to which a blocking voltage and/or a forward current can be applied by a controller, so that the detuning is realized by a changed junction capacitance or by a switching action in combination with a series or parallel connected capacitance and/or inductance.

Here, combinations of elements of the detuning device 51 and other circuits are also conceivable. For example, the active detuning 52 and the passive detuning 53 may also be implemented separately without a common path or component. In particular, the method according to the invention and the magnetic resonance tomography apparatus according to the invention solve the technical problem of being able to check the function of as many components or all components as possible.

An embodiment of the method according to the invention is schematically shown in a flow chart in fig. 4.

With the method according to the invention, it is provided that in step S5 the detuning device of the antenna coil, for example the local coil 50, is activated by the test controller. The test controller can be a controller 23 of the magnetic resonance tomography apparatus 1, which is implemented by a corresponding program or program modules. But also special purpose processors or logic circuits, such as FPGAs, are conceivable. When activated, the active detuning 52 is controlled such that the local coil 50 or, for example, the body coil 14, which is electrically connected to the active detuning 52, no longer has resonance at the frequency optimal for the reception situation in the activated state. In general, the optimum frequency is the larmor frequency of the nuclear spins to be examined in the static magnetic field B0 of the magnetic resonance tomography apparatus 1. In different embodiments of the method according to the invention, the activation of the active detuning 52 can take place at different points in time, not just from the beginning as shown in fig. 3.

The different embodiments of the method according to the invention have in common that the second received signal is received with the receiver at two different points in time in step S20 and step S50. Here, for example, the other steps of the method are performed between steps S20 and S50, or at least the received signal is considered to be a different point in time due to a change in physical conditions (e.g. due to an attenuated or reduced excitation) between steps S20 and S50. Therefore, the two time points of steps S20 and S50 differ by more than 1ms, 5m, 10ms, 50ms, or 100ms, for example.

In step S60, the verification controller compares the first received signal with the second received signal. For example the amplitudes or the squares of the amplitudes of the two received signals form a difference, which can be regarded as a comparison. It is also conceivable to form phase differences or to perform a spectral analysis of the energy distribution over a plurality or a plurality of frequencies. The function applied for this, e.g. forming a logarithm, may be part of the comparison.

Finally, in step S70, if the result of the comparison does not coincide with the predetermined relationship of the first received signal with respect to the second received signal, a warning signal is output. Various examples of signals and desired predetermined relationships are given below. The warning signal can be output via an output to the operator or can also be output directly to the control unit 20, so that it interrupts further image acquisition, for example, in order to avoid endangering the patient 100 due to a faulty detuning device 51.

Step S70 can also be regarded here as an extended implementation of step S30 in fig. 2, in which the predetermined impulse response is a received further check pulse.

In one embodiment of the method according to the invention, it is provided that in step S30, the detuning device is deactivated by the test controller. Thus, one of the two receive signals is received with active detuning 52 activated, while the other receive signal is received with active detuning 52 deactivated. Step S10 may be performed before step S20, for example, and S30 may be performed between steps S20 and S50. It is provided that the magnetic resonance tomography apparatus 1 does not transmit a useful signal during steps S20 and S50, in other words, the radio-frequency unit 22 and/or the test transmitter 60 are not controlled by an input signal. The received signal is therefore characterized by a noise signal, for example of the output stage of the transmitter. In step S60 of the comparison, the noise level of the first received signal is thus compared with the noise level of the second received signal. It is conceivable, for example, by means of the active detuning 52 that the level and the energy of the received signal received by the receiver 65 change in a characteristic manner. This may be related to or caused by a changed resonance frequency of the antenna or local coil 50. It is therefore also conceivable to take into account, for comparison, for example by means of an FFT, changes in the spectral distribution of the noise signal, which are derived from the convolution of the spectrum of the noise source with the resonance frequency of the antenna, which changes in turn by active detuning. In an advantageous manner, the amplitude of the noise signal is so small that no passive detuning 53 occurs, so that only the function of the active detuning 52 can be checked.

However, it is also conceivable in an embodiment of the method according to the invention for the transmitter of the magnetic resonance tomography apparatus to transmit a small signal during the receiving steps S20, S50, which small signal is designed to, but not to, control the receiver.

The transmitter can be, on the one hand, a transmitter which is used by the magnetic resonance tomography apparatus to generate excitation pulses for the nuclear spins during image acquisition. However, transmitters are typically designed to generate extremely strong high frequency pulses in the range of hundreds of watts to thousands of watts with high efficiency. In order to generate a small signal which does not override the receiver 65, the magnetic resonance tomography apparatus may have a linear output stage, and switchable attenuation means between the signal generator and the linear output stage which attenuate the input signal of the linear output stage sufficiently during the receiving steps S20, S50 to generate a corresponding small signal at the output of the linear output stage. In principle, it is also conceivable to attenuate the output signal of a conventional power output stage, but in this case a high power must be derived as a power loss of the attenuation element.

For example, the generated small signals can be transmitted into the patient channel 16 by the body coil 14 as a transmitting antenna, with the local coil 50 as a receiving antenna.

A separate test transmitter 60 is also conceivable as part of the magnetic resonance tomography apparatus 1 according to the invention, which test generator generates a small signal under the control of the test controller and emits it via the transmission antenna 70 into the patient tunnel 16 with the local coil 50.

By the power and/or frequency spectrum varying between step S20 and step S50, the active generation of small signals gives more freedom in comparing the first received signal and the second received signal in the method according to the invention. For example, the power of the small signal may vary greatly between steps S20 and S50, so that the passive detuning 53 already participates in the case of a larger power and limits the level of the received signal. The function of the passive detuning 53 can be checked by means of this limitation or the non-linearity in the case of three or more measurement values.

It is also conceivable to change the spectral distribution of the small signals between the steps in order to check the function of the active detuning 52 and/or the passive detuning 53. Different combinations of different small signals and different settings of the active detuning are conceivable here, which check all the functions of the detuning means 51.

It is also conceivable that the nuclear spins themselves act as small signal sources after excitation. For example, measurements from a previous image acquisition sequence may be used to estimate the strength of the desired magnetic resonance signal and its temporal course. For known spin densities and excitation signals, Bloch-Simulation of the desired magnetic resonance signal can be conceived. A typical trend here is an exponential decrease in the magnetic resonance signal. The steps already described can also be carried out with such small signals having a known temporal course, in particular a check for active detuning 53, which does not require a minimum amplitude for the small signal in response to the detuning compared to passive detuning.

The nuclear spins can also be provided by the phantom. At low signal intensities, it is also conceivable to use the patient as a source of magnetic resonance signals. However, passive resonators, such as oscillating circuits or dipoles, which are excited by the excitation pulse of the high-frequency unit 22 and have an exponential signal drop predetermined by the quality of the oscillating circuit or dipole, are also conceivable.

Although the invention has been illustrated and described in detail by means of preferred embodiments, the invention is not limited to the disclosed examples and other variants can be derived therefrom by the person skilled in the art without departing from the scope of protection of the invention.

Claims

1. A method for operating a magnetic resonance tomography apparatus, wherein the magnetic resonance tomography apparatus (1) has a controller (23), a local coil (50) and a transmitter, which is designed to generate excitation pulses for nuclear spins during image acquisition and to generate and transmit examination pulses,

wherein the method has the following steps:

the input signal is generated by a signal generator of the transmitter,

attenuating the test pulse by attenuation means of the transmitter, so that a power output stage of the transmitter outputs a predetermined test pulse with a reduced power,

(S10) transmitting a predetermined check pulse with reduced power by the transmitter;

(S20) receiving the inspection pulse with the local coil (50);

(S30) comparing, by the controller (23), the received verification pulse with a predetermined impulse response;

(S40) outputting a warning signal by the controller (23) when the received check signal deviates from a predetermined impulse response.

2. The method of claim 1, wherein the predetermined verify pulse has an amplitude of 0 volts and the predetermined impulse response has an elevated noise level.

3. The method of claim 1, wherein the amplitude of the verify pulse increases from a lower starting value to a higher ending value over the duration of the verify pulse.

4. The method of claim 3, wherein the local coil has detuning means and the detuning means of the local coil are activated during the receiving step.

5. The method of any of claims 1-4, wherein the predetermined impulse response has a threshold.

6. The method of any of claims 1 to 4, wherein the predetermined impulse response is proportional to the verify pulse.

7. Method according to claim 1 for checking detuning means (51) of an antenna coil of a magnetic resonance tomography apparatus (1), wherein the method has the following steps:

(S5) activating detuning means (51) of the antenna coil;

(S20) receiving the first reception signal with the receiver (65);

(S50) receiving a second reception signal with the receiver (65);

(S60) comparing the first received signal with the second received signal by the inspection controller;

(S70) if the result of the comparison does not coincide with the predetermined relationship of the first received signal with respect to the second received signal, outputting a warning signal.

8. The method according to claim 7, wherein the method has a step (S25) of deactivating the detuning means (51), wherein the step (S50) of receiving the second reception signal is performed with the detuning means (51) switched off, and wherein in the step of comparing (S60) the noise level of the first reception signal is compared with the noise level of the second reception signal.

9. The method of claim 8, wherein a transmitter of the magnetic resonance tomography apparatus transmits a small signal during the receiving step (S20, S50), the small signal being designed not to over-control the receiver (65).

10. The method of claim 9, wherein no input signal is fed to the power output stage of the transmitter during the receiving step (S20, S50).

11. The method of claim 7, wherein during the receiving step (S20, S50), a signal source is arranged within the patient passageway (16) and a signal is transmitted, wherein the power of the signal varies by a predetermined amount between the step of receiving the first received signal (S20) and the step of receiving the second received signal (S50).

12. The method according to claim 11, wherein a transmitter of the magnetic resonance tomography apparatus (1) generates a signal as a signal source and the signal is a small signal which does not control the receiver (65), and the check controller controls the transmitter to change the power in step (S40).

13. The method of claim 11, wherein the signal source is a passive signal source, the signal source being excited by the magnetic resonance tomography apparatus (1) to output a small signal, wherein between the step of receiving a first received signal (S20) and the step of receiving a second received signal (S50), the amplitude of the small signal emitted by the signal source decreases over time in a predetermined manner.

14. A magnetic resonance tomography apparatus, wherein the magnetic resonance tomography apparatus (1) has a controller (23), a local coil (50), a transmitter and a transmission antenna for transmitting excitation pulses, the transmitter having a power output stage and a switchable attenuation device which is designed to attenuate an input signal of the power output stage,

wherein the controller (23) is designed to transmit a predetermined test pulse with reduced power by means of the transmitter and the transmitting antenna,

wherein the controller (23) is further designed to receive the test pulse by means of the local coil (50);

wherein the controller (23) is further designed to compare the received test pulse with a predetermined impulse response; and is

When the received test signal deviates from the predetermined impulse response, a warning signal is output.

15. The magnetic resonance tomography apparatus of claim 14, wherein the transmitter has a small signal path enabling a small signal to be fed directly into the transmitting antenna.

16. The magnetic resonance tomography apparatus of claim 15, wherein the small signal path provides a direct connection between the high frequency source of the transmitter and the transmit antenna bypassing the power output stage.

17. The magnetic resonance tomography apparatus as set forth in claim 16, wherein the small signal path has a small signal amplifier.

18. The magnetic resonance tomography apparatus as claimed in one of claims 15 to 17, wherein the controller (23) is designed to transmit a predetermined test pulse with reduced power via the small signal path and the transmit antenna;

wherein the controller (23) is designed to compare the received test pulse with a predetermined impulse response; and is

19. The magnetic resonance tomography apparatus of claim 15, wherein the magnetic resonance tomography apparatus has a directional coupler and a test switch, which is in signal connection with the small-signal path and the directional coupler, wherein the controller is designed to feed the test signal from the small-signal path via the test switch into the directional coupler and to compare an output signal of the directional coupler with a predetermined value.

20. The magnetic resonance tomography apparatus as claimed in claim 14, having an examination apparatus for examining a detuning apparatus (51) of an antenna coil of the magnetic resonance tomography apparatus (1), wherein the examination apparatus has an examination controller and the examination controller is designed for activating the detuning apparatus (51) of the antenna coil;

receiving a first received signal with a receiver (65);

receiving a second received signal with the receiver (65);

the first received signal is compared with the second received signal and if the result of the comparison does not correspond to the predetermined relationship of the first received signal relative to the second received signal, a warning signal is output.

21. The magnetic resonance tomography apparatus as claimed in claim 20, wherein the magnetic resonance tomography apparatus (1) has a transmitter for generating the excitation pulses, which transmitter has a switchable attenuation device, and the examination controller is designed to transmit a small signal by means of the transmitter and the attenuation device during the step (S20, S50) of receiving the first and/or second receive signal, which small signal is designed not to control the receiver (65).

22. The magnetic resonance tomography apparatus as claimed in claim 20, wherein the magnetic resonance tomography apparatus (1) has an active signal source which is arranged in the patient tunnel (16) and transmits a signal during the receiving step (S20, S50), wherein the test controller is designed to vary the power of the signal by a predetermined variable between the step of receiving a first received signal (S20) and the step of receiving a second received signal (S50).

23. A computer-readable storage medium having electronically readable control information stored thereon, which control information is designed to carry out the method according to one of claims 1 to 6 when the storage medium is used in a controller (23) of a magnetic resonance tomography apparatus (1) according to claim 14.